Alkyl phosphonic dihalides have been prepared in the prior art by reacting alkyl halides with phosphorus tri-halides in the presence of aluminum chloride. The reaction proceeds at room temperature according to the formula set forth in Heuben-Weyl, Methoden der Organis Chenchemie at Volume 12, Part 1 (1965) at page 396: ##STR1## The Heuben-Weyl reference also notes that the reaction has been attempted in the absence of the aluminum chloride catalyst with little success.
Phosphorus pentoxide and phosphorus pentasulfide are known reactants in the field of organophosphorus chemistry as evidenced by U.S. Pat. Nos. 2,471,472, 2,482,810, 2,495,799, and 2,685,602. It is also known (U.S. Pat. No. 2,622,917, Example IV) that phenyldichlorophosphine can be reacted at a moderate or relatively low temperature in the presence of an aluminum chloride catalyst with phosphorus trichloride and phosphorus pentasulfide to provide benzenethionophosphonyl dichloride (C.sub.6 H.sub.5 P(S)Cl.sub.2). Removal of the catalyst provides significantly decreased yields. This patent also teaches that benzene can be reacted with phosphorus trichloride in the presence of aluminum chloride and phosphorus pentasulfide at moderate temperatures (40.degree. C.), cf. Ex. VII.
It is also known (U.S. Pat. No. 2,882,303) that alkyl phosphonyl dihalides, i.e., methane phosphonyl dichloride can be prepared in an autoclave from methyl chloride, phosphorus trichloride and a phosphoryl compound ##STR2## such as phosphoryl trichloride using phosphorus diiodide as catalyst.
It is also known that chlorobenzene will react with phosphorus pentoxide at an optimum temperature of 275.degree. C. at a reaction time of 24 hours (Lecher et al, Am. Chem. Soc. 76, Pages 1045-1051, 2/20/1954). The product obtained is an anhydride of phosphorus having the formula ##STR3## as well as a polymeric anhydride of the phosphonic acid corresponding to the formula (ClC.sub.6 H.sub.4 PO.sub.2)x.